The electrical cable consists of an electrically highly conductive metallic conductor, an inner conductive layer that surrounds the metallic conductor, an insulating layer arranged above the inner conductive layer, an outer conductive layer that surrounds the insulation, a sheath made of insulating material, which has been rendered electrically conductive, arranged above the outer conductive layer, and an electrically conductive intermediate layer arranged between the outer conductive layer and the sheath (DE 19638603 AI).
Linear motors have long been known for electric drives of various types. In this regard, there are both direct-current and alternating-current synchronous and asynchronous motors. In contrast to a conventional motor, in a linear motor, both a stationary stator and a moving rotor are arranged linearly rather than circularly. The electrical energy is converted to mechanical energy in a linear motor in such a way that it can be used directly for a translational motion. Fields of application for linear motors are passenger vehicles, conveyance and transportation, conveyor lines, luggage conveyance, mining, cranes, towing equipment, machine tool carriages, and the operation of valves. In principle, the linear motor can have a field winding that is arranged in grooves of an inductor and can have a three-phase design in the case of alternating current. The rotor portion then consists either of a rail of an electrically highly conductive material, such as copper or aluminum (asynchronous motor), or of permanently magnetic material (synchronous motor).
If a linear motor of this type is used, for example, to drive a high-speed long-distance maglev train, the inductor and thus the cable installed in its grooves are then very long. Since for this reason a linear motor of this type is operated at a relatively high voltage, the cable must be equipped with an inner and an outer conductive layer as well as a shield. The shield of medium-voltage cables of this type is necessary for safely carrying capacitive charging currents, for ground fault detection, for allowing the possibility of fault location, and as protection against mechanical damage to the layers surrounding the conductor. In addition, it is intended to protect living beings from being endangered by high voltages.
When medium-voltage cables with the structure described above are used in the very long stator (consisting of the inductor and cables) of a linear motor, a high longitudinal voltage is induced in the shields of the cable, which can amount to well over 1 kV for a stator 100 m long. To prevent such high voltages from arising, the shields could be divided into very short segments, and each segment could be single-ended, i.e., grounded on one side. This is complicated and expensive and increases the risk of cable faults. With previously known shielded medium-voltage cables, low shield voltages could also be achieved by the grounding of the shields on both ends of a segment of almost any length or by connection of the shields of the three cables used for the winding in almost any intervals. However, large shield currents would then flow, which would cause large energy losses and would act as an eddy-current brake.
DE 30 06 382 A1 describes a three-phase alternating-current winding for a linear motor that consists of medium-voltage electrical cables with the structure described above. The cables used here have an outer sheath that consists of an insulating material that has been rendered conductive. On at least one side of the stator, a strand of an electrically highly conductive material is arranged in the area of the winding heads that extend out of the grooves. This strand extends the whole length of the stator, is in good contact with the conductive sheaths of the cables, and can be connected to ground potential. The electrically conductive sheaths of the cables simultaneously constitute their shield, which has a relatively low electrical conductivity. The combination of the sheaths with the strand connected to ground potential results all together in a shield that guarantees good diversion of capacitive currents and also ensures that currents arising as a result of induced voltages remain small. All together, the winding thus has low dissipation, and the influence on the field becomes negligible. Moreover, since high voltages cannot arise, endangerment of living beings is avoided.
In the previously known cable described in DE 196 38 603 A1, which was cited above, a metal mesh is present as an intermediate layer that is closed all around and extends the whole length of the cable between the outer conductive layer and the sheath, which has been rendered electrically conductive. This intermediate metal mesh increases the electrical conductivity of the shield of the cable. The purpose of this is to minimize electric voltages and currents that are induced in the shield and could diminish the driving power of the linear motor. In addition, the metal mesh is intended to make the axial resistance along the axis of the cable homogeneous and to ensure the detection of a ground fault and the drainage of fault currents more easily and to a sufficient extent. Indeed, this is achieved with this previously known cable in many applications. Nevertheless, it can happen that the cable is damaged, especially at elevated operating voltage.